This is just our experience, maybe we just got lucky?
We put our privacy fence up 22 years ago, pressure treated posts. It's dark and I can't go out and measure their size right now.
We used a 4 ft hole, about 14 inches diameter, with about 2 ft covered with concrete. I check the stability of the fence weekly, and if there are holes being dug, because of the dogs. Right now, the posts don't budge.
About 8 years ago this area had a tornado. The next day I looked out and there was a string that I can best describe as the kind of string TheHusbandHalf uses, with blue chalk, to mark a straight line, so a substantial string, that was 30 ft in the air. It was going across the whole yard, and was held by the trees.
Then we noticed one of the fence posts was leaning in.
It was being held up by the fence panels it was attached to. The post must have sustained some strong wind, but there was no actual tornado here. Where the post broke, there was no sign of rot, it was like when we put it in.
We have a high water table here, especially out back
Like I said, maybe we were just lucky!